The invention relates to a method of copying transparent masters and to a photographic copier.
Individual regions of photographic images often have large brightness variations so that finer structures are no longer observable in the darker regions. This is due to the small dynamic range of the photosensitive copy paper. In contrast, on the photographic film from which the paper copies are produced, the fine structures are also present in the darker regions because of the greater dynamic range.
U.S. Pat. No. 4,239,385 describes a copier in which the contrast between the light and dark regions is reduced to such an extent that the brightness gradations in the original can be adjusted to the dynamic range of the photosensitive copy paper. To this end, a liquid crystal display is placed between the light source and the photographic film. A black-and-white negative mask of the transparent original is generated in this display via an electric control unit. When exposure of the copy paper is now carried out via the mask an the original, a desired density compression is obtained.
In a similar apparatus shown by U.S. patent application Ser. No. 08/480,283, a mask representing a very blurry inverse image of the transparent original is produced in an LCD display. By virtue of this blurriness, only the so-called large area contrast is reduced which the contrast of details is maintained.
However, it has been surprisingly found that, even with this high degree of blurriness, the superposition of the mask and the original plays a large role. Since, under certain circumstances, the individual pixels of the LCD display become visible on the paper image, these are additionally blurred by a diffusing disk between the display and the transparent original. As a result, however, the distance between the mask and the original is small which negatively affects the quality of the paper images.